catch IO errors when writing media files, closes jgm#4559

if we do not catch these errors, any malformed entry in a media bag
could cause the loss of a whole document output. an example of
malformed entry is an entry with an empty file path

src/Text/Pandoc/Class.hs

@@ -477,6 +477,14 @@ liftIOError f u = do
          Left e  -> throwError $ PandocIOError u e
          Right r -> return r
 
+-- | Show potential IO errors to the user continuing execution anyway
+logIOError :: IO a -> PandocIO ()
+logIOError f = do
+  res <- liftIO $ tryIOError f
+  case res of
+    Left e -> report $ IgnoredIOError (E.displayException e)
+    Right _ -> pure ()
+
 instance PandocMonad PandocIO where
   lookupEnv = liftIO . IO.lookupEnv
   getCurrentTime = liftIO IO.getCurrentTime
@@ -862,7 +870,7 @@ writeMedia dir mediabag subpath = do
        Just (_, bs) -> do
          report $ Extracting fullpath
          liftIOError (createDirectoryIfMissing True) (takeDirectory fullpath)
-         liftIOError (\p -> BL.writeFile p bs) fullpath
+         logIOError $ BL.writeFile fullpath bs
 
 adjustImagePath :: FilePath -> [FilePath] -> Inline -> Inline
 adjustImagePath dir paths (Image attr lab (src, tit))

src/Text/Pandoc/Logging.hs

@@ -85,6 +85,7 @@ data LogMessage =
   | InlineNotRendered Inline
   | BlockNotRendered Block
   | DocxParserWarning String
+  | IgnoredIOError String
   | CouldNotFetchResource String String
   | CouldNotDetermineImageSize String String
   | CouldNotConvertImage String String
@@ -175,6 +176,8 @@ instance ToJSON LogMessage where
            ["contents" .= toJSON bl]
       DocxParserWarning s ->
            ["contents" .= Text.pack s]
+      IgnoredIOError s ->
+           ["contents" .= Text.pack s]
       CouldNotFetchResource fp s ->
            ["path" .= Text.pack fp,
             "message" .= Text.pack s]
@@ -265,6 +268,8 @@ showLogMessage msg =
          "Not rendering " ++ show bl
        DocxParserWarning s ->
          "Docx parser warning: " ++ s
+       IgnoredIOError s ->
+         "IO Error (ignored): " ++ s
        CouldNotFetchResource fp s ->
          "Could not fetch resource '" ++ fp ++ "'" ++
            if null s then "" else ": " ++ s
@@ -332,6 +337,7 @@ messageVerbosity msg =
        InlineNotRendered{}          -> INFO
        BlockNotRendered{}           -> INFO
        DocxParserWarning{}          -> INFO
+       IgnoredIOError{}             -> WARNING
        CouldNotFetchResource{}      -> WARNING
        CouldNotDetermineImageSize{} -> WARNING
        CouldNotConvertImage{}       -> WARNING